Tenacity and Mercury's Treads

No, I'm not dissing Merc treads, I'm just curious as to why merc treads are the only item that you can buy for permanent Tenacity ( doesn't count. It's not permanent) I remember back from season 3-5 we had Zephyr which was really good if you need to buy or something. The way it is currently is a little frustrating considering that are the only permanent tenacity in the game. I know you can get some tenacity from masteries, but that's only 15%. I understand that it is frustrating to play against someone who would stack tenacity. cough cough But in order to do that, these characters have to sacrifice other stats to do so. I'm just kind of thinking out loud here so if anyone else has any input I'd be interested to hear it.